Many utility lines such as gas, electricity and water lines are buffed several feet below the surface of the earth. In order to service such utility lines, the ground above them must first be excavated. In cold climates, the ground freezes solid to depths of three to six feet, creating a cement-like barrier between the surface of the ground and the underground lines. Although it is possible to use pneumatic hammers to excavate the frozen ground, pneumatic hammers are labor intensive and the shock waves generated by the hammers may damage the very underground line for which access is sought. Also, in some jurisdictions it may be illegal to use power digging tools in the vicinity of oburied utilities. It is desirable to first defrost the frozen ground, and then manually excavate the ground.
U.S. Pat. No. 5,033,452 (Carriere) discloses a thawing device having a thermally insulated housing and a single undivided fire tube mounted within the housing. The fire tube has a first end connected to one port in the housing and a second end connected to another port in the housing. A burner is mounted in the first end of the fire tube, and a flue for exhausting the combustion gases is connected adjacent the second end. Heat radiates from the fire tube into the interior of the housing. The housing includes a pitched roof having a steam vent at its apex that extends between the interior and the exterior of the housing. The purpose of the steam vent is to collect and discharge steam generated during the thawing process within the housing. Some of the heat in the housing unavoidably escapes with the steam and air that is vented.
U.S. Pat. No. 4,349,010 (Bentham) discloses a frost removal system in which heating pipes are placed on the ground to be thawed and then covered with earth. In one embodiment, a generally cylindrical chamber includes a tangential inlet where a burner is mounted and a centrally disposed exhaust outlet extending upwardly through the earth covering.
Another ground thawing device, called "Frost Hog," is manufactured by Leric Holdings, Ltd., of Lloydminster, Alberta, Canada. The device includes a heavy trailer-mounted housing and a fire tube extending through the housing from one port to another port. A burner is positioned in the first port and a vertical flue for exhausting the combustion gases is positioned adjacent the second port. This device is said to burn approximately 200 pounds of propane to defrost the ground to a depth of four to five feet in about 24 hours.
Yet another ground thawing device, called the "Thawmaster," is manufactured by Thawmaster of Minneapolis, Minnesota. This device uses heating tubes that are disposed directly in holes that, in a preliminary step, are bored in the ground.
Desirably, a thawing device should be manually portable, require a minimum of site preparation, and be safe and efficient in operation.